1. Field of the Invention
The present invention relates to an amplifier for an optical receiver circuit, and particularly to an amplifier which can stably maintain a constant output amplitude by automatically and instantaneously controlling offset voltage and gain in response to wide range of changes in an input signal.
2. Description of Related Art
Amplifiers are essential in an optical digital communications system for amplifying an attenuated signal in an optical receiver circuit, or for generating a stable clock signal by extracting a timing signal from input data by using a timing signal recovery circuit. These amplifiers are required to have a large gain, a wide dynamic range for an input signal, and a constant output amplitude.
In an optical receiver circuit for an optical digital communications system as shown in FIG. 1, an optical signal is first converted into a current signal by a photodiode. Subsequently, the current signal is converted into a voltage signal, and then, amplified by an amplifier to a voltage amplitude, at which the logic levels can be identified. In addition, the voltage amplitude of the output signal must be maintained constant by automatically controlling the offset voltage and gain of the amplifier even if the input signal changes its level. A conventional amplifier implements these functions by combining a variable gain amplifier and a DC feedback circuit.
Recently, introduction of an optical communications system that handles a burst mode data signals of various signal levels has been studied. The optical receiver circuit of such a system strongly requires an amplifier that can maintain a constant output amplitude by instantaneously controlling the offset voltage and gain.
The conventional amplifier, however, has a problem in that it is slow in response because of a feedback loop, and cannot follow changes in the input signal. This problem will be explained in detail referring to FIG. 2 showing a conventional optical receiver circuit.
In this figure, the intensity of an optical signal is converted into a current by a photodiode 1, and the current signal produced from the photodiode 1 is amplified and converted into a voltage signal by a preamplifier 2. The voltage output from the preamplifier 2 is further amplified and converted into an output voltage of a fixed amplitude by a voltage amplifier 3 which automatically performs the offset compensation and the gain control. The output voltage of a fixed amplitude is produced from the output terminal To.
The offset compensation refers to a function which is achieved by providing the DC bias of the amplifier with an offset so that the reference voltage of the voltage amplifier 3 (that is, the voltage at point d of FIG. 2) is always set at the midpoint of the input signal waveform WF, which is shown by a dashed-and-dotted line in FIG. 2. Thus, the voltage amplifier 3 operates in the linear operating region whose center is the reference voltage.
For example, if the peak value (referred to as a top value below) of the output waveform WF from the preamplifier 2 varies as shown in FIG. 2 in response to changes in the optical digital signal inputted to the photodiode 1, and is amplified by a common amplifier, only half of the linear operating region of the amplifier (one direction with regard to the bottom value of the waveform) becomes available. Thus, a wide dynamic range cannot be efficiently achieved, and distortions of the output waveform and deviations of the duty thereof are liable to occur. In view of this, the reference voltage applied to the voltage amplifier 3 is continually controlled to always take the middle value of the waveform WF by providing the DC bias of the amplifier with an offset. Thus, the voltage amplifier 3 operates in the linear operating region whose center is determined at the renewed reference voltage.
More specifically, the offset control is carried out by an automatic offset compensation control circuit 5 including a top-holding capacitor 5A and a voltage amplifier 5B. The automatic offset compensation control circuit 5 detects the top value (peak value) of the output voltage of a variable gain amplifier 4, and feeds the detected component back to the offset control terminal d of the variable gain amplifier 4. Thus, the output waveform of the variable gain amplifier 4 is controlled to take a constant top level.
On the other hand, the gain control refers to a function that controls the gain of the voltage amplifier 3 in order to maintain its output amplitude even if the input signal changes its amplitude or level.
More specifically, the gain control is achieved by a gain control circuit 6 including a top-holding capacitor 6A and a voltage amplifier 6B. The gain control circuit 6 detects the top value of the output voltage of the variable gain amplifier 4, and feeds the detected component back to the gain control terminal c of the variable gain amplifier 4. Thus, the output amplitude of the voltage amplifier is controlled to take a constant value.
Combining these two feedback circuits can implement the offset compensation and the gain compensation at the same time. Thus, the automatic offset compensation control circuit 5 and the gain control circuit 6 have a top-detecting circuit composed of an amplifier and a capacitor. The top-detecting circuit charges the capacitor when the input is high, and blocks the discharge path by establishing a high output impedance of the amplifier when the input is low, thereby holding the detected top level of the waveform.
FIG. 3A schematically illustrates an input waveform to the conventional voltage amplifier, and FIG. 3B illustrates the output waveform of the voltage amplifier produced in response to the input waveform.
When the input waveform to the amplifier 4 changes instantaneously from a small to a large amplitude as illustrated in FIG. 3A, the output waveform begins to be generated under the condition that the small amplitude operation has established with regard to the gain and offset voltage, because the offset compensation and the gain compensation cannot be carried out instantaneously. As a result, the output waveform as illustrated in FIG. 3B is produced. In this case, a desired output amplitude cannot be obtained until the feedback fully achieves its function. The response delay is determined by the time constants associated with the capacitors 5A and 6A of the top-detecting circuits (that is, the automatic offset compensation control circuit 5 and the gain control circuit 6), and the delay times of the negative feedback circuits and the amplifier.
The time constants associated with the capacitors 5A and 6A of the top-detecting circuit must be reduced as small as possible, in order to implement a circuit which provides a desired output amplitude as quick as possible. Even this, however, cannot give a response characteristics quicker than the delay time of the amplifier. Furthermore, although the time constant of the automatic gain control circuit 6 and that of the automatic offset compensation control circuit 5 are set different from each other to achieve a stable operation, the difference between the two must be reduced to implement the instantaneous response. This would make the system unstable because of the interference between the two feedback loops. This instability of the system occurs because the offset and the gain of the voltage amplifier are not independent.
As described above, the output waveform of the voltage amplifier employed by the conventional optical receiver circuit largely changes in response to an instantaneous, large change in the input signal amplitude, and it takes rather a long time for the output waveform to take a constant amplitude. Thus, the conventional voltage amplifier is unsuitable to a receiver circuit that must achieve an instantaneous response to amplitude or level changes in a received optical signal. In other words, the conventional voltage amplifier has a problem in that the amplitude and duty of the output waveform are unstable because of the delay of the feedback circuits, when the amplitude or level of the input signal changes instantaneously.